The approval of new drugs have slowed down significantly in the last couple of decades, not because of a slow down in the discovery of new therapeutic agents (e.g., small molecule drugs, proteins) with good activity in vitro (good pharmacodynamics), but because most of those new agents have poor efficacy in vivo (poor pharmacokinetics) in humans. Out of more than 6,000 new molecules synthesized in the pharmaceutical industry, only one gains FDA approval to be used in the clinic and only one in twenty drug candidates that make it to clinical trials reaches the market. To make matters worse, it takes longer than a decade to develop a promising drug candidate and the expense of doing so has been estimated to reach up to $1 billion. Faced with this situation, most pharmaceutical companies decide to narrow their focus towards diseases that afflict only large segments of the population, leaving many other human diseases untreated. There is a critical need to solve this problem in order to improve human health and quality of life. A number of potential solutions have been proposed, including making and screening even larger libraries of compounds and using novel therapeutic agents (e.g., proteins, DNA and RNA) for alternative treatment strategies (e.g., gene delivery, interfering-RNA). Those strategies, however, will face similar obstacles to small-molecule based drugs, which is, unacceptable in vivo efficacy due to poor pharmacokinetics. One of the most promising strategies to overcome this problem, however, relies on the use of nanoparticles (NPs) as Drug Delivery Systems (DDS). The field of nanoparticle-based drug delivery systems (NP-DDS) is vast and with a relatively long history spanning more than four decades. We describe NPs, with a size less than 10 μm, in the context of systems with the capability to contain or encapsulate from a variety of guest small molecules to complex macromolecules by protecting them from the outer environment (e.g., physiological conditions). By protecting the encapsulated cargo from the surroundings, NPs can be targeted as a DDS to a current tissue with a specific disease. A number of such DDS are already used in the clinic to treat diseases such as cancer, while others are currently undergoing clinical trials. The most successful DDS to date rely on nanoparticles based on lipids (LNP) and polymers (PNP) have continued to be used as DDS.
Although such LNPs and PNPs are indeed providing steady progress towards improving drug efficacy, they have certain limitations. LNPs tend to be unstable during dilution (e.g., upon administration), which lead to leakage of the encapsulated drug. PNPs have overcome many of the limitations of LNPs, but they suffer from other issues such as polydispersity (i.e., a distribution of molecules of similar but not identical properties), which become more prevalent upon scaling up their synthesis, making it difficult to obtain the high purity and reproducibility required to make clinically grade materials. However, such limitations regarding scaling up in synthesis, purification and reproducibility could be overcome by exploring the use of small molecules. Despite all the work done in the field, to the best of our knowledge (and besides lipids), there are no examples of small-molecule NP-DDS.
A convenient way to develop small-molecule NP-DDS is the use of supramolecular chemistry, which enhances the construction of complex chemical systems performed via non-covalent interactions (e.g., hydrogen bonds, π-π interactions, metal coordination, ion pairing, hydrophobic effects, van der Waals forces). In comparison with molecules that use atoms as building blocks by covalent bonds, in supramolecular systems, molecules are the building blocks of the resulting non-covalent assemblies. With the advantage of using supramolecular chemistry, we have been synthesizing hydrophilic 8-aryl-2′deoxyguanosine (8ArG) derivatives and studying their self-assembly in order to expand the current knowledge of the self-assembly of small molecules in aqueous media and to test the potential for the corresponding supramolecules to serve as self-assembled ligands for G-quadruplex DNA (QDNA). Guanosine is a naturally occurring recognition motif well known for its ability to pair with cytosine in both DNA and RNA structures. Guanosine is also self-complementary, which allows it to form planar cyclic tetrameric structures that further associate into coaxial stacks to form either supramolecular G-quadruplexes (SGQs, made from individual G subunits) or oligonucleotide G-quadruplexes (e.g., QDNA, QRNA). QDNA is the subject of intense studies due to its putative role in telomere function and in the regulation of some oncogenes.
This library of hydrophilic 8ArG derivatives, which have allowed us to deepen the fundamental knowledge of small molecule self-assembly in water, provided structural insights of the resulting supramolecular G-quadruplexes (SGQs). For example, we have developed the 8-meta-acetyl-guanosine (mAG) moiety into an attractive recognition motif to enable the construction of well-defined and discrete supramolecular nanostructures. We demonstrated this by using it to construct lipophilic hexadecameric self-assembled dendrimers (SADs). Our discovery that a positively charged hydrophilic mAG derivative self-assembles isostructurally into hexadecamers in aqueous media prompted us to explore the construction of congeneric hydrophilic SADs. In addition, we have developed ways to modulate the supramolecular properties such as molecularity, fidelity and stability (thermodynamic and kinetic). We have evaluated how changing parameters that are intrinsic (structure of the derivatives) and extrinsic (e.g., pH, temperature, concentration, cation template) to the SGQs alter their structure and dynamics.
While studying the self-assembly of some of the 8ArG derivatives described before, we discovered one particular derivative whose corresponding SGQ showed thermoresponsive behavior. We demonstrated that upon reaching a threshold temperature (Lower Critical Solution Temperature, LCST) these SGQs further self-assemble to form nano/micro hydrogel globules we termed supramolecular hacky sacks (SHS). The LCST is a phenomenon observed in amphiphilic systems that are soluble at one temperature, but once a transition temperature is reached, there is a volume phase transition that produces a change in the solvation state from random coil to globule (e.g., polymeric systems). This represented, to the best of our knowledge, the first example of thermoresponsive system made from a non-polymeric precise supramolecule. Such LCST phenomenon enables the construction of “smart” NP-DDS whose formation can be triggered by just increasing the temperature above its transition temperature.
These findings uncovered a new paradigm in the development of smart responsive materials with properties and applications similar to those of polymeric systems, but with the potential advantage of being based on small molecules. Since that initial discovery, we further reported these SHS to be suitable for encapsulating the anti-cancer drug doxorubicin and, more recently, we reported on ways to modulate the LCST by altering the distribution of hydrophobic patches on the surface of the SGQs. However, biorelevant applications of thermoresponsive materials are limited by the narrow window of physiological temperatures, which complicates their implementation.
Nevertheless, the human body has different regions (organs) that have different pH values (human tumors 5.7-7.8, endosome pH<6), making a pH stimulus a feasible strategy for the design of pH responsive materials for drug encapsulation, biosensors and other novel materials in biotechnology. Currently, in the field of pH-responsive materials, most reported systems are polymeric, such as micelles, liposomes, dendrimers, hydrogels, and others. Despite the increasing number of reported stimuli responsive polymeric materials and nanoparticles, there are still significant challenges that must be overcome, such as difficult synthetic protocols, cytotoxicity and complex modifications for biological recognition.
Self-assembly is a very convenient tool for the construction of pH-responsive supramolecular nanomaterials. Even though this method offers an advantage over conventional organic multistep synthesis, there are not many reported examples of non-polymeric pH-responsive materials. Even with the existence of many reported thermo- and pH-responsive systems that can self-assemble under a certain stimulus; there are still certain limitations regarding the precise modulation of the composition and structure of these materials.